The advice, given to MPs today, also refutes suggestions that unwilling clergy might be forced by human rights law to marry same-sex couples.

The Equality and Human Rights Commission has dismissed claims that legalising same-sex marriage will lead to discrimination against people who continue to believe that marriage can only be between a man and a woman. The advice, given to MPs today, also refutes suggestions that unwilling clergy might be forced by human rights law to marry same-sex couples. Any such attempt, it concludes, would be "extremely likely to fail."

Parliament is beginning its detailed consideration of the bill today.

Ever since the government announced its intention to change the law, opponents have argued that people who take a more traditional view of marriage will face discrimination in the workplace, even potentially losing their jobs for expressing their beliefs.

A letter organised last month on behalf of Catholic priests and bishops (more than a thousand signed it) compared the prospect to the situation their church faced after the Reformation, when Catholics were legally barred from holding many official positions. The move, the priests predicted, "will have many legal consequences, severely restricting the ability of Catholics to teach the truth about marriage in their schools, charitable institutions or places of worship. It is meaningless to argue that Catholics and others may still teach their beliefs about marriage in schools and other arenas if they are also expected to uphold the opposite view at the same time."

Similar fears have been expressed by other campaigners. The Conservative MP Edward Leigh introduced a Ten Minute Rule bill at the end of January calling for explicit protection to be given to opponents of same-sex weddings in churches - by making the exclusively heterosexual view of marriage a "protected characteristic" under the 2010 Equality Act. Without such protection, he warned, "Army and NHS chaplains who preach in favour of traditional marriage in their own churches on Sunday could find themselves in trouble," while "tens of thousands" of teachers could face disciplinary action.

Today's advice from the EHRC, written by a leading QC, suggests that these fears are misplaced. When it comes to religious ceremonies, it notes that "freedom to manifest religion or belief" is enshrined in the Human Rights Act, as well as in Article 9 of the European Convention. The principle is not absolute, since a government can interfere with it in the wider public good, but in this case the government has said very clearly that it wishes to uphold the right of religious objection. Churches and other religious bodies will be able to opt-in to performing same-sex marriages, but that will be entirely their choice.

The EHRC also sees "no reason why employees of all kinds will not remain free to express their views about same-sex marriage." They, too, would enjoy the full protection of Article 9. Furthermore, the Equality Act itself protects employees from direct and indirect discrimination, and also unfair dismissal, because of their religion or belief. Employees should not be sanctioned for disagreeing with the new law, since it "would be unlawful for an employer to discipline or sack an employee for this. This is the case for all employees, whether in the public or private sector, including teachers and chaplains." Nor would be anyone be required to promote same-sex marriage as part of their job.

The guidance concludes that there "is sufficient protection for individuals who hold the religious or philosophical belief that marriage should only be between a man and a woman." The only exception the EHRC can see is that registrars might be required to officiate at same-sex weddings as part of their public duty: but as the recent case of Lillian Ladele showed, this is already true of civil partnership ceremonies.

Campaigners against the Bill will probably dismiss this advice as speculative. Seemingly contradictory advice from the human rights lawyer Aidan O'Neill was publicised last month in the Telegraph. Nevertheless, such a clear statement from the EHRC is likely to carry weight, since it has a statutory duty to scrutinise legislation and to issue formal advice to employers. The advice on same-sex marriage comes on the day that the Commission also circulates new guidance on the wider question of the expression of religion and belief in the workplace, which it hopes will avoid conflict and costly court cases.

If there was a danger of over-zealous employers interpreting the new law as requiring staff to suppress their opposition to same-sex marriage, today's strong advice from the EHRC makes such a scenario much less likely.